Question

I have some opaque bytes which I want to use in an std::map, both as keys and values. I created a class, OpaqueBytes, which has two (private) members: _data and _size, to store the bytes. My Cocoa background tells me that I would need to implement a hashing function and equality function on the class to use it in a std::map. Problem is, it seems like the STL requires strict weak ordering, based on Can a STL map be used with keys of varying sizes.

I have several questions:

1. Is my intuition to create a class to represent opaque bytes correct? Is there a class in the STL which already exists to hold opaque bytes? In Cocoa parlance, is there an equivilent to NSData?

2. How should I implement the ordering of OpaqueBytes? I can’t just compare the bytes using memcmp, because the two byte strings could be of differing length – I wouldn’t want FF EE to be considered equal to FF.

Answer 1

If you need to use OpaqueBytes as a key you will need a strict comparison. This can be a member function or a non-member function.

Could use something like this:

// assumed a friend
bool operator<( OpaqueBytes const& lhs, OpaqueBytes const& rhs )
{
   int res = memcmp( lhs._data, rhs._data, std::min(lhs_._size, rhs_._size)  );
   return ( res < 0 ) || ( res == 0 && lhs._size < rhs._size );
}

You could compare size first, of course. You’d get a different ordering but it would still be strict.

(I still think, as an aside, that map would be better implemented with a compare function than a less).